FIELD OF THE INVENTION
The present invention relates to a manhole cover frame for mounting a manhole cover above a manhole basin. The frame allows adjustment of the height of the manhole cover as necessitated by resurfacing a road surface and/or by frost heave of the surrounding ground.
Manhole basins, which may be a sewer basin having an inlet and an outlet, or may be a catchment basin having only an inlet, are normally cast of concrete. A frame of metal is fixed to the top of the basin and holds the manhole cover. In order to keep the cover level with a road or ground surface it is frequently necessary to alter the height between the manhole cover and the top of the basin, particularly in regions where there is much frost heave; adjustment to the height is usually done when a road is resurfaced.
Various constructions of adjustable manhole frames have been proposed to deal with the adjustment and to avoid replacing concrete parts. In some cases, screw jacks have been used between the annular frame which holds the cover and the top of the concrete manhole basin. In others, an outer ring, supported by the basin, has internal screw threads of large diameter within which is an inner ring having mating screw threads, and which is rotatable to adjust its height. Examples of patents showing such systems are:
U.S. Pat. No. 3,533,199, which issued Oct. 13, 1970 to Pickett;
U.S. Pat. No. 3,629,981, which issued Dec. 28, 1971 to McCaffrey;
U.S. Pat. No. 3,930,739, which issued Jan. 6, 1976 to Larsson et al.;
U.S. Pat. No. 4,075,796, which issued Feb. 28, 1978 to Cuozzo;
U.S. Pat. No. 4,149,816, which issued Apr. 17, 1979 to Piso;
U.S. Pat. No. 4,925,237, which issued May 15, 1990 to Spiess et al., and
U.S. Pat. No. 5,095,667, which issued Mar. 17, 1992 to Ryan et al.
These prior arrangements have some drawbacks. Where, as in the Pickett, McCaffrey, Cuozzo and Ryan et al. patents, reliance is placed on large diameter screw threads between outer and inner rings, these threads become worn and the manhole cover then starts to wobble as vehicles pass over, leading to more wear and possible breakage. The prior art systems using screw jacks have limited range of adjustability since screw jacks are likely to become unstable if extended too far.